1. Field of the Invention
The present invention relates to a method of finishing an edge of a sheet glass for use in e.g. fire-resistant construction material such as a fire-resistant door, a fire-resistant window or the like, a heat-tempered sheet glass using the method, and relates also to a fire-resistant construction material using this heat-tempered sheet glass.
2. Description of the Related Art
When a sheet glass is used in a fire-resistant construction material such as a fire-resistant door, it is necessary for the glass to have a high edge strength in order to avoid a heat cracking phenomenon of this sheet glass. Namely, when a sheet glass is subjected to a significant heat, heat expansion occurring at the center area of the sheet glass applies a tension to the edge of the sheet glass fixedly supported to a window sash, for example. Then, when this tension exceeds the edge strength of the sheet glass, a cracking occurs in the sheet glass. This is the heat cracking phenomenon.
As fire-resistant sheet glass, there are known a wired sheet glass and a laminated glass incorporating an intermediate layer made of silica hydrate alkaline. In the case of the former, its transparency may be a problem since the embedded wire hinders the the view. The latter also has the possibility of losing its transparency due to bubbling of the intermediate layer resulting from a thermal variation. As an alternative free from such drawbacks, there is known a sheet glass prepared by heat-tempering treatment of soda-lime glass material.
As such heat tempering treatment, the convention has provided a method comprising the steps of: heating a sheet glass at a temperature range (approximately, 760 deg. C.) significantly higher than a melting point of the glass (720 to 730 deg. C.); and then spraying a cooling air against the sheet glass at an extremely high pressure such as a back pressure of 950 mmAq, for instance. With such heat tempering treatment of the sheet glass as above, it is possible indeed to provide the sheet glass with a predetermined edge strength. However, since the air is sprayed at the very high pressure against the sheet glass which has been heated up to such high temperature range as exceeding the melting point of the glass, this spraying may result in surface irregularity or warping of the sheet glass which leads to distortion of the image reflected by the sheet glass.
According to a conventional method of finishing an edge of such sheet glass as described supra, in order to obtain a flat and even edge, as illustrated in FIGS. 6(a) and (b), a cup abrading wheel 20 is rotated about an axis while causing a side face 20a of the wheel to contact an edge 21a or a ridge 21b of the sheet glass 21. The cup abrading wheel includes diamond or abrading stone or the like affixed on the side face 20a thereof.
With such sheet glass edge abrading method as described supra, a very find abrade streak, which is hardly recognizable with naked eyes, is formed at the ridge portion as a result from the abrading of the edge portion. For this reason, a thermal stress associated with a thermal expansion tends to be concentrated at the abrade streak. In particular, in the case of the soda-lime glass (without heat-tempering treatment), it is not possible to provide this glass with a high edge strength.
Accordingly, in order to allow its use as a fire-resistant sheet glass, the above-described heat tempering treatment is needed. But, this treatment involves the inconveniences described supra. Further, if the heat tempering treatment is effected at a lower heating temperature and/or lower back pressure of the spraying air than the conventional method in order to avoid the inconveniences (surface irregularity, warping(of the heat tempering treatment, then, this will result in an insufficient edge strength of the sheet glass obtained as a fire-resistant door.
In view of the above-described state of the art, the primary object of the invention is to solve the drawbacks of the conventional art by providing an improved method of finishing an edge of a sheet glass capable of increasing the edge strength of the glass, a heat-tempered glass with a predetermined edge strength made by using the method, and also a fire-resistant construction material using this heat-tempered sheet glass.